Thermal imaging or thermography is a recording process wherein images are generated by the use of thermal energy.
In thermography three approaches are known:
1. Direct thermal formation of a visible image pattern by image-wise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
2. Image-wise transfer of an ingredient necessary for the chemical or physical process bringing about changes in colour or optical density to a receptor element.
3. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an image-wise heated donor element onto a receptor element.
Thermographic materials of type 1 become photothermographic upon incorporating a photosensitive agent which after exposure to UV, visible or IR light is capable of catalyzing or participating in a thermographic process bringing about changes in colour or optical density.
WO 94/16361 discloses a multilayer heat-sensitive material which comprises: a color-forming layer comprising: a color-forming amount of finely divided, solid colorless noble metal or iron salt of an organic acid distributed in a carrier composition; a color-developing amount of a cyclic or aromatic organic reducing agent, which at thermal copy and printing temperatures is capable of a color-forming reaction with the noble metal or iron salt; and an image-toning agent; characterized in that (a) the carrier composition comprises a substantially water-soluble polymeric carrier and a dispersing agent for the noble metal or iron salt and (b) the material comprises a protective overcoating layer for the color-forming layer.
WO 97/04355 discloses a photothermographic recording material comprising a support and a photo-addressable thermally developable element comprising photosensitive silver halide in catalytic association with a substantially light-insensitive silver salt of an organic carboxylic acid, an organic reducing agent for said substantially light-insensitive silver salt of an organic carboxylic acid in thermal working relationship therewith and a binder, characterized in that said binder, comprises a non-proteinaceous water-soluble binder, a non-proteinaceous water-dispersible binder or a mixture of a non-proteinaceous water-soluble binder and a non-proteinaceous water-dispersible binder.
U.S. Pat. No. 4,708,928 discloses a photothermographic active particle having dimensions between 0.5 and 100 microns comprising a transparent binder, photosensitive silver halide, light insensitive silver compound, and a reducing agent for silver ion.
EP-A 736 799 discloses a recording material comprising a support having provided thereon at least a recording layer comprising (a) a heat-responsive microcapsule having encapsulated therein an organic silver salt, (b) a developer for the organic silver salt and (c) a water-soluble binder.
The inventors of the present invention found that thermographic and photothermographic recording materials coated from aqueous media using the teachings of U.S. Pat. No. 4,708,928 and EP-A 736 799 with reducing agent in close proximity to the organic silver salt in particles or microcapsules exhibited poor archivability and poor light stability.
The poor archivability and poor light stability of thermographic and photothermographic recording materials coated from aqueous media is a general problem and has led to most commercial thermographic and photothermographic materials being coated from solvent media despite the obvious economic and environmental disadvantages thereof. There is therefore a need for thermographic and photothermographic recording materials coatable from aqueous media which exhibit comparable or better stability than recording materials coated from solvent media.